A touch panel can include a plurality of drive electrodes (or “drive lines”) and a plurality of sense electrodes (or “sense lines”), wherein the drive electrodes are separated from the sense electrodes by a dielectric material. For example, the drive electrodes and the sense electrodes can be embodied as thin and relatively narrow, elongated electrical traces disposed on opposite sides of a dielectric substrate. The drive electrodes may be oriented in a first direction and the sense electrodes may be oriented in a second direction so that the drive electrodes and sense electrodes intersect without touching. The drive electrodes and sense electrodes form capacitive sensors at the points of intersection.
The drive electrodes and sense electrodes are coupled to a control circuit as would be known to those skilled in the art. The control circuit periodically applies a strobe signal to one of the drive electrodes while at the same time tying the rest of the drive electrodes to ground. The strobe signal generates an electric field about the drive electrode. This electric field couples to the sense electrodes about the sensor locations formed by the drive electrode and the intersecting sense electrodes, thereby establishing mutual capacitances between the drive electrode and the sense electrodes (“drive-sense mutual capacitance”) at each of these sensor locations.
The foregoing drive-sense mutual capacitances will have a steady state value in the absence of a stimulus proximate the respective sensor locations. Introduction of a stimulus, for example, a finger or other conductive object, proximate a particular sensor location can result in a portion of the electric field about that sensor location coupling to the stimulus, thereby establishing a mutual capacitance between the drive electrode and the stimulus at that sensor location. This phenomenon lessens the drive-sense mutual capacitance at that sensor location.
The control circuit detects the drive-sense mutual capacitance at each of the sensor locations. The control circuit distinguishes between the steady state drive-sense mutual capacitance at each of the sensor locations and the lessened drive-sense mutual capacitance resulting from introduction of a stimulus (if any) proximate the sensor location. The control circuit provides an output indicative of the presence or absence of a stimulus proximate a sensor location based on the drive-sense mutual capacitance at that sensor location.
In embodiments wherein the drive electrodes are relatively narrow traces, the density of the electric field about a strobed drive electrode is fairly uniform over the width of the drive electrode. In such embodiments, the manner in which the electric field couples to a stimulus proximate a strobed sensor is not significantly affected by the location of the stimulus with respect to the width of the drive electrode.
In some embodiments, the drive electrodes can be relatively wide. In such embodiments, the density of the electric field about a strobed drive electrode can vary significantly from the centerline of the drive electrode to the edges of the drive electrode, the electric field density generally being substantially greater about the centerline of the drive electrode than about the edges of the drive electrode. Accordingly, the proportion of the electric field that couples to a stimulus proximate a strobed drive electrode can vary substantially depending on the location of the stimulus with respect to the width of the drive electrode. A greater proportion of the electric field will couple to a particular stimulus located proximate the centerline of the drive electrode than to the same stimulus if proximate an edge of the drive electrode. As such, the drive-sense mutual capacitance at that sensor will be lower when a stimulus is introduced proximate the centerline of the electrode than if the same stimulus were introduced proximate an edge of the drive electrode.
The control circuit can distinguish between the lessened drive-sense mutual capacitance resulting from introduction of a stimulus proximate a sensor location about the centerline of the respective, relatively wide drive electrode and the lessened drive-sense mutual capacitance resulting from introduction of a stimulus proximate a sensor location about the edges of the respective, relatively wide drive electrode. Based on this distinction, the control circuit can provide an output indicative of whether the stimulus is proximate the centerline of the drive electrode or proximate the edges of the drive electrode.
To the extent that the control circuit deems the stimulus to be proximate an edge of the drive electrode, the control circuit, without more, cannot distinguish whether the stimulus is proximate one edge or the other. The control circuit, however, can subsequently strobe an adjacent drive electrode and detect the drive-sense mutual capacitances at the corresponding adjacent sensor. If the drive-sense mutual capacitance at the adjacent sensor is at steady state, indicating the absence of a stimulus there, the control circuit may deem the stimulus to be proximate the opposite edge of the previously-strobed drive electrode. If the drive-sense mutual capacitance at one of the adjacent sensors is less than steady state, indicating the presence of a stimulus there, the control circuit may deem the stimulus to be proximate the adjacent edge of the previously-strobed drive electrode.